The present invention is applicable to Wavelength Division Multiplexing (WDM) optical fiber communication, and it relates to a wavelength-selection optical switch for switching between transmission and blocking of a light signal with a certain wavelength, included in a light signal propagating with a plurality of different wavelengths inside an optical fiber, and an Optical Add/Drop multiplexer (OADM) of wavelength selection type for addition (insertion) and/or drop (extraction) of a light signal with a certain wavelength.
In the field of Wavelength Division Multiplexing communication, a wavelength-selection optical attenuator for extracting or inserting a light with a certain wavelength is one of the important devices. As the wavelength-selection optical attenuator, there is a multi-channel wavelength-selection optical attenuator which separates a multiplexed wavelength light exiting from an optical fiber to free space by using a grating (diffraction grating), after attenuating (blocking) a light with a certain wavelength by using a spatial light modulator such as a liquid crystal device, adds again the other light by using a grating, and then directs again to an optical fiber. Further, it is also known that a variable optical attenuator (so-called “wavelength-blocker”) can appropriately vary a wavelength to be selected.
FIGS. 7 (a) and (b) illustrate views schematically showing the exemplified configuration of the variable optical attenuator (wavelength blocker) of the prior art.
FIG. 7 (a) shows a wavelength blocker supplied by Xtellus Inc. (U.S.) described in the non-patent document 1. Light exited from a Single Mode Fiber (SMF) 1001 at the input side is collimated by an optical lens 1002, after orienting polarization directions to a certain direction by using a polarization separator or polarization coupler 1003 fabricated using such as calcite, and then separated by a grating 1004. The selected light is focused to a LCD (Liquid Crystal Device) spatial light modulator 1006 by using an optical lens 1005 aligned in the focal plane, and attenuated by a liquid crystal shutter (or a micro-machine shutter) of this LCD spatial light modulator 1006, and thereby eliminated.
Then, the selected light is guided to a single mode optical fiber 1007 at the output side via an optical system symmetrical to that of input side.
Also, FIG. 7 (b) shows a wavelength blocker supplied by Optogone Inc. (FR) described in the non-patent document 2. Although its configuration is essentially the same as the one described in the aforementioned non-patent document 1, the wavelength blocker is made simply by altering the LCD spatial light modulator 1006 to that of the reflection type, and separating the input from the output by using a circulator 1008.
On the other hand, heretofore, an optical fiber which selectively reflects light with a certain wavelength from a wavelength multiplexed light has been known. References such as patent document 1 propose an external-cavity semiconductor laser with a variable wavelength optical filter obtained by using a liquid crystal device. Since the variable wavelength optical filter using the liquid crystal device works by applying a desired voltage, the semiconductor laser includes no mechanical parts, resists the temperature variation and mechanical vibration, has an advantage in the stability of an oscillation wavelength, duty cycle, polarization plane, etc., and operates stably for a long time.
FIG. 8 illustrates views schematically showing the configuration of a variable wavelength optical filter 100 of the prior art: (a) a longitudinal sectional view, and (b) top view.
The variable wavelength optical filter 100 comprises a silicon substrate 112, a glass substrate 121 aligned so as to face the surface of this silicon substrate, with a transparent electrode 119 on the opposing surface. Further, a light diffraction reflection layer 125 is arranged on the surface of the silicon substrate, and a liquid crystal layer 117 is arranged between the light diffraction reflection layer 125 and transparent electrode 119. The liquid crystal layer 117 is sealed by a sealing wall 118 formed around it.
The light diffraction reflection layer 125 includes, in order from the silicon substrate 112, a cladding layer 113, diffraction grating 114, and optical guide layer 115.
In addition, the silicon substrate 112 and glass substrate 121 are covered their respective surfaces with antireflection coatings 111a, 111b and 120a, 120b. Also, each surface of the transparent electrode layer 119 and the optical guide layer 115 contacting with the liquid crystal layer 117 is covered with an alignment film 116a, 116b. 
With this configuration of the variable wavelength optical filter, it is possible to control the reflection index by varying the orientation of the liquid crystal layer 117 through changing the voltage applied between the silicon substrate 112 and the transparent electrode layer 119, and as a result vary the wavelength of the light being diffracted and reflected.
As shown in FIG. 8 (a), in a side surface shown in the longitudinal sectional surface, a glass substrate side electrical terminal 123 is extracted from the transparent electrode layer 119 formed on the glass substrate 121, and, in the other side surface, a silicon substrate side electrical terminal 122 is extracted from the front surface of the silicon substrate 112, i.e., the surface of the light diffraction reflection layer 125 side.
Regarding the extraction of the glass substrate side electrical terminal 123, in order to ensure the width w1 for extracting, the glass substrate 121 projects relative to the silicon substrate 112.
On the other hand, regarding the extraction of the silicon substrate side electrical terminal 122, in order to ensure the width w2 for extraction, the silicon substrate 112 projects relative to the glass substrate 121. Although, heretofore, the silicon substrate 112 and light diffraction reflection layer 125 have been manufactured by using the conventional processes for manufacturing the silicon device, the silicon substrate side electrical terminal 122 can only be extracted from the front surface of the silicon substrate 112 in order to ensure the consistency of the manufacture processes.
The concrete reason is as follows. Heretofore, in the process for forming the silicon substrate side electrical terminal 122, alignment marks (such as projection and concave) formed on the front surface of the silicon substrate is used (see patent documents 2, 3, etc.). These alignment marks are utilized for a variety of processes such as a process for forming patterns on a surface of the silicon substrate. On the other hand, since the thickness of the silicon substrate is thick, it is difficult to detect the alignment marks formed on the front surface by the observation from the rear surface using a visual light. Therefore, heretofore, it has been inevitable to extend the silicon substrate side electrical terminal 122 from the front surface of the silicon substrate 112.
However, in recent years, in order to perform FMEA (Failure Mode and Effect Analysis) of the semiconductor devices formed on the silicon substrate, a method was proposed for detecting marks such as alignment marks formed on the front surfaces of the silicon substrate by projecting an infrared beam from the rear surface (see patent documents 4, 5, etc.).
In the patent document 4, an apparatus is disclosed which has a light source outputting an infrared beam with a wavelength region transmitting through the silicon substrate, captures images of a first reflection beam from the rear surface when projecting an output beam from the light source to the rear surface of the substrate and a second reflection beam from inside the substrate and the front surface reflected after the transmission into the substrate, and obtains observed data on the reflection position by numerically processing each image data.
Also, in the patent document 5, observation of semiconductor devices formed on the front surface from rear surface side is realized by using a silicon substrate, through which infrared transmits, and separately aligning semiconductor devices, which work as alignment means, formed on surface of the substrate such that each separation is less than or equal to half of the minimum observation scope of a FMEA system and more than or equal to 3 semiconductor devices are aligned in the observation scope.
[Patent document 1] Japanese Patent Application Laid-Open Publication No. 05-346564
[Patent document 2] Japanese Patent Application Laid-Open Publication No. 10-209009
[Patent document 3] Japanese Patent Application Laid-Open Publication No. 2005-340321
[Patent document 4] Japanese Patent Application Laid-Open Publication No. 2005-221368
[Patent document 5] Japanese Patent Application Laid-Open Publication No. 2005-311243
[Non-patent document 1] S. Patel and Y Silberberg “Liquid crystal and Grating-based multiple-wavelength cross-connect switch,” IEEE PTL vol. 7, pp. 514-516 1995
[Non-patent document 2] “Dynamic spectral equalizer using free-space discursive optics combined with a polymer-dispersed LC spatial light attenuator,” IEEE JLT vol. 21 pp. 2061002073, 2003
Although the wavelength blocker described in the aforementioned non-patent document 1 has good extinction properties with more than or equal to 40 dB, the light signal to be blocked is merely extinguished and impossible to be extracted outside. Namely, it doesn't work as an optical switch which can adequately switch between transmission and block of a signal with a certain wavelength. In addition, since many elements are included and their positioning is time consuming, the blocker is not practical.
The wavelength blocker described in the non-patent document 2 can't work as an optical switch, as it cannot extract a light signal to be blocked. Therefore, in order to compose the optical add/drop multiplexer, an add port of 3 dB coupler and drop port of 3 dB become necessary, and thus the configuration of the circuit is complicated, and the add/drop multiplexer is not practical for use.
In addition, since the variable wavelength optical filter of the prior art, as shown in the top view in FIG. 8 (b), has a large height (short side of a rectangular shape) of about 5 mm and a wide width (long side of the rectangular shape) of about 7 mm, the housing size of the external-cavity semiconductor laser obtained by using this variable wavelength optical filter is large with a height of about 12 mm and a width of about 16 mm.
Presently, a next generation optical communication system for 10 GHz communication has been developed, however, a transponder to be applied to its Truck line is designed compact, the size of a housing of the external-cavity semiconductor laser to be assembled in the system is restricted by the size of the transponder used in the optical communication system, and is inevitable to be less than 9 mm in height and 13 mm in width. In addition, in order to operate the semiconductor laser efficiently, it is used under the temperature control at a temperature of about 50 deg., and it becomes inevitable to make the size of the housing of the external-cavity semiconductor laser less than the aforementioned size, which is also necessary to increase the efficiency of the temperature control. Further, it is necessary to ensure spaces inside the housing of the external-cavity semiconductor laser, for setting a Peltier device for temperature control and a stem for mounting parts, extracting electrodes to be connected to such as a semiconductor laser and variable wavelength optical filter, and fixing these parts. By considering the thickness of the housing, a height of no more than 3 mm and a width of no more than 4.5 mm are strongly desired for the variable wavelength optical filter.
However, as shown in FIG. 8 (a), the variable wavelength optical filter 100 of the prior art is formed by sealing the liquid crystal layer 117 with the sealing wall 118 between the silicon substrate 112 and the glass substrate 121, and the width of the sealing wall 118 must be more than or equal to 0.5 mm by the restriction from manufacturing technology. In addition, by the restriction from the technology of cutting or cleavage, more than or equal to 0.2 mm is necessary for a width of a margin for cutting, from the sealing wall 118.
Further, due to manufacture technology restrictions for connecting a copper wire of, for example, 0.5 mm diameter to the silicon substrate side electrical terminal 122 and glass substrate side electrical terminal 123, more than or equal to 0.6 mm is necessary for the width of the electrical terminal, and due to restrictions in the technology of the cutting or cleavage of a glass substrate and silicon substrate, more than or equal to 0.2 mm is necessary for the width of margins from the silicon substrate side electrical terminal 122 and glass substrate side electrical terminal 123. In this case, more than or equal to 0.8 mm is necessary for both widths w1 and w2 shown in FIG. 8 (a).
Hereupon, if the silicon substrate side electrical terminal 122 is extracted from the rear surface of the silicon substrate 112, the width w2 shown in FIG. 8 (a) becomes unnecessary, however, it is extremely difficult to extract an electrical terminal from the rear surface of the silicon substrate 112 as mentioned above.
Moreover, by the restriction from the fact that the effective area width (to be described below referring to FIG. 3) along the plane of the light diffraction reflection layer 125 is shaded by the sealing wall 118, more than or equal to 0.2 mm is necessary for a gap between the effective area and the sealing wall 118.
Therefore, in order to satisfy the aforementioned entire dimensional requirement, a height of at least 3.3 mm and a width of at least 5.3 mm are necessary for the conventional variable wavelength optical filter.
As described hitherto, with a conventional configuration of the variable wavelength optical filter, it is extremely difficult to miniaturize to the sizes with a height less than or equal to 3 mm and a width less than or equal to 4.5 mm. On this, although it was tried to simply reduce the width of the sealing wall to 0.2 mm, the manufacturing yield dropped owing to extreme reduction of the strength of the sealing wall, and thereby the optical filter was not practical to use.
The present invention is performed by considering the aforementioned problem of the prior art, and the objective of the present invention is to provide a wavelength-selection variable optical switch which can variably attenuate a light with a certain wavelength, has a switching functionality which is able to extract a light to be extinguished, and is miniaturized to the size with a height of less than or equal to 3 mm and a width of less than or equal to 4.5 mm. In addition, it is also the objective to provide an optical add/drop multiplexer by using this optical switch.